Capacitive affinity sensors have been used to measure the concentration of an analyte by detecting a change in capacitance as molecules move in or out of an electric field between two electrodes of the sensor, for instance. The movement of molecules onto and off of the surface changes the dielectric properties of a biochemically active layer between the two electrodes. The displacement of the solvent molecules by the moving molecules reduces the measured capacitance between the two electrodes. The capacitance between the two electrodes changes in relation to the concentration of the analyte being measured by such a sensor, for instance. Such capacitive affinity sensors, however, have a sensitivity limited by the amount of water displaced from the sensor surface by normal biomolecules.
U.S Pat. No. 4,728,882 to Stanbro et al. describes a capacitive sensor for determining the presence of volatile materials such as hydrocarbons in a liquid medium. The sensor of that patent includes a concentrating layer of room temperature vulcanized (RTV) silicone rubber having a high affinity for non-polar molecules. Hydrocarbon molecules are non-polar and readily enter the surface of the concentrating layer of silicone rubber. As a result, bubbles nucleate on the surface of the concentrating layer of silicone rubber in proportion to hydrocarbon concentration with a corresponding reduction in capacitance of the sensor. The sensor of the Stanbro et al. Patent measures an amount of molecules of a volatile material, but does not include a means for generating molecules of such a volatile material. The sensor of the Stanbro et al. Patent is, therefore, not as versatile as the present inventor's biochemical sensor. The present inventor is a co-inventor of the Stanbro et al. Patent.
U.S. Pat. No. 3,817,837 to Rubenstein et al. describes a method for assaying concentrations of organic materials according to enzymatic activity. An amplification is obtained by forming a large number of molecules in the presence of one molecule. This patent also describes specific methods of assaying for an enzyme, including the use of ion specific electrodes. No mention is made, however, of nucleated bubbles, of a surface upon which bubbles might nucleate, or of a sensor that responds to such nucleated bubbles.
A need exists for a biochemical sensor that has an increased sensitivity facilitated by a bubble mechanism, where a volatile material is generated that nucleates as bubbles and comes out of solution into the gas phase at the surface of the sensor.